Aerodynamic stability of the downstream of two tandem square-section cylinders

Abstract The work reported in the present paper consists of three parts. In part one, the velocity distribution in the wake of a square cylinder at different distances from it (2⩽ x / D ⩽12) are measured and reported. Analytical expressions for the wake velocity distribution and for the correlation between wake half-width and downstream distance are obtained. The above expressions make it possible to estimate the wake velocity distribution without the availability of the actual experimental data. In part two, the lift and drag acting on the downstream of two cylinders are measured. The results are found to be in reasonable agreement (except in the range L / D =3–4 and T / D =2–3) with previous measurements, and are presented as contours of constant quantities, which make them useful to other researchers for quick information retrieval or estimation. Based on these steady flow results, the region where the downstream cylinder will become unstable to transverse galloping (static instability) are estimated and reported. In the next part, data are acquired with the downstream cylinder undergoing transverse oscillation. From the measurement of the phase angle between the body frequency component of the lift force and the cylinder displacement, the region where the downstream cylinder will be (dynamically) unstable to transverse galloping is estimated, and is found to be in good agreement with the estimation based on the steady flow results in the range L / D ⩽4. The variations of the mean drag as well as the Strouhal number and fluctuating lift and drag of the downstream cylinder with reduced velocity are also measured at different L / D and A / D , and possible explanations for the behaviour of the data are offered.